Numerous conventional packages for integrated circuit (“IC”) devices involve the formation of solder bumps or other suitable contacts onto associated I/O contact pads formed on an active face of a die. The die is then typically attached to a suitable substrate, such as a printed circuit board (“PCB”), such that the solder bumps on the die are surface mounted to contact pads on the substrate. The solder bumps are then typically reflowed to form solder joint connections between the substrate and the attached die. Such a surface mount device (“SMD”) is often referred to as “flip chip,” since the chip or die must be “flipped” to place its active surface containing solder bumps into contact with the substrate to which the chip is to be attached.
Many problems associated with connecting a solder bump to a contact pad, such as issues with wetting, adhesion, temperature cycling induced stresses, and the electrical isolation needs, among others, are solved by providing additional items between the solder bump and the contact pad. Such items can include, for example, passivation layers, resilient layers and under bump metallization (“UBM”) stacks, among others. For purposes of efficiency, these items are all typically formed onto dice at the wafer level prior to separation, with such formation processes being generally known to those skilled in the art. Contact pads are usually formed first onto an active surface of a wafer, with one or more passivation and/or resilient layers having individual vias corresponding to individual contact pads coming next. UBM stacks having a multiple layers are formed above and typically through the vias, such that the bottom layer of each UBM stack contacts a contact pad or an electrical connector to a contact pad. A common UBM stack formation includes a bottom layer of aluminum for adhering to a contact pad, a top layer of copper for wetting to a solder bump, and a middle layer of nickel-vanadium or other suitable metal or alloy for adhering copper to aluminum.
Although the provision of UBM stacks solves many problems associated with connecting solder bumps to contact pads, other problems are introduced through their use. For example, a typical UBM stack is commonly formed by conventional sputtering and etching processes such that the end portion of each layer is exposed at the vertical edges of the UBM stack. Although an underfill layer may provide some protection, many SMDs now forgo the use of underfill layers, rendering their UBM stack edges as permanently exposed. For aluminum in particular, these exposed stack edges can experience corrosion and lead to eventual failure whenever moisture or other contaminants are introduced. Contamination may occur, for example, due to contact with a poorly formed solder paste or the inadequate cleaning of an associated pressure chamber having undesirable residues. In addition, the use of conventionally sputtered and etched layers frequently results in sharp edges, which can exacerbate stresses due to temperature cycling in any layer placed directly atop a sharp edge.
Although existing UBM structures work well in many situations, there are thus ongoing efforts to further improve the UBM structures used in various integrated circuit devices such that problems like those give above are minimized or eliminated.